The Mitogen-Activated Protein kinases (MAPs) have been recently the focus of intensive study. This family of homologous kinases is involved in a variety of cellular responses to extracellular stimuli and their respective activation status appears to be determinant for the cellular fate. The identification of distinct MAPK cascades, consisting in at least 3-kinase modules, well conserved between all eukaryotes, has partly enlighted the respective panel of responses involving each of the kinase signaling pathway. The ERKs module is activated by a mitogen or a differentiation signal and in turn, activates their substrates that include p90 ribosomal S6 kinase, cPLA2, PHAS-1, c-myc, MAPKAPK2 and Elk1. On another hand, cellular responses to stresses, to some growth factors, pro-inflammatory cytokines, UV or γ-radiation, ceramides, vasoactive peptides, protein synthesis inhibitors or heat shock involve the activation of the Jun N-terminal Kinases (JNKs) and of the p38s/HOG. The end points of this stress kinase cascades are the phosphorylation of the c-Jun, Elk1 or ATF-2 (CRE-BP1) transcription factors. The persistent activation of the JNKs is associated with growth arrest, occurrence of apoptosis or activation of the hematopoietic cells.
The JNKs are activated by dual phosphorylation by the JNK kinases (MKK4/SEK1) which are, in turn, activated by upstream serine threonine kinases referred to as MEK kinases (MEKKs). MEKKs represent an expanding family of kinases. Mammalian MEKK1 cDNA encodes a protein of 78 kDa but several forms of MEKK1 were reported in various cell lines (50, 78 or 98 kDa). Thereafter, a rat MEKK1 full-length cDNA was cloned, encoding a 195 kDa protein. This was reported to be cleaved by a caspase, resulting in the expression of a shorter and more active kinase during anoikis (apoptosis due to cell detachment). The 98 kDa cleavage product corresponds to the 625 aminoacids at the C-terminal part of the full-length MEKK1. Recent datas have demonstrated that MEKK1 regulates also the nFκB transcription factor, by phosphorylating the iκB-α kinase.
The wide range of extracellular or intracellular stimuli leading to activation of the MAPKs raises the question of the specificity of their mechanism of activation. The activation steps of the mitogen kinase cascade (ERKs) have been described. In contrast, the first activating steps that regulate the stress kinase cascade are at the moment unknown. Common regulators of this MAPK/JNK cascade have been identified, such as phosphatases, and act as determinants of the balance between cellular growth and apoptosis, balance that regulates the homeostasis of all tissues.
Both Raf (the MAPKKK that is activated by mitogenic, Ras-dependent signals) and MEKK1 interact with GTP-bound Ras through its effector domain but there is no evidence that Ras-GTP activates directly the MEKK1 protein. Nevertheless, oncogenic Ras activates also the JNK cascade, and this activation seems to be necessary for Ras transformation.
These examples illustrate that MEKK1 participates to the regulation of a wide range of cellular events leading either to cell division, to cell activation or to cell death. Thus, a striking control of its activity should exist in the cells. An increase of MEKK1 activity could lead to an excess of apoptosis or T cell activation, and be at the origin of a wide range of pathologies as inflammation and asthma, immunosuppression, cardiac ischemia or hypertrophy, myelodysplasic syndromes, neurodegenerescence etc. On the other hand, a down-regulation of the MEKK1 activity could induce an excess of cell proliferation and/or survival and therefore lead to tumor growth, excessive angiogenesis, rheumatoid arthritis, psoriasis and sustained viral infections.
A cDNA that is 100% homologous to the MIF1 cDNA disclosed herein was published by Ren Y. et al., Eur. J. Biochem., 253, pp734-742, 1998, acc Genbank AF015308, and named Homo sapiens microspherule protein (MSP58). No function was assigned to this protein, however.
However, there is a need in the art to better understand the molecular mechanisms of MEKK-mediated cellular processes. In particular, there is a need in the art to identify a MEKK regulatory protein.
The present invention addresses this need, as discussed below.
The citation of any reference herein should not be construed as an admission that such reference is available as “Prior Art” to the instant application.